Thread: D2 tyre pressures

I've come across a couple of web pages that take a more hands on approach to the adjusting pressures using contact patches. The idea here is that you set your front pressures, then measure the length of the contact patch. You then adjust the rear pressures so the contact patch length matches the fronts.

A post on ExplorOZ includes screenshots of the Leyland Brothers using a stick marked at various lengths to adjust pressures based on contact patch length.

The method is also described here: Direct 4WD - Tours in the Australian Outback and here:
The Lowdown, Off Road Tire Pressures | ARB USA Blog

This looks to be far simpler to use than trying to calculate correct pressures for the rear based on loading, and gives the same outcome.

Add:

I just noticed the ExplorOZ thread has a post from a guy (Olsen's 4WD Tours and Training) that uses the axle loading method to calculate correct starting pressures and then uses the contact patch method to adjust for other conditions. I'd been adding 2 on the calculated result, while he suggests adding 4 psi.

Originally Posted by biggin

Offtrack's threads have finally convinced me to drop some pressure from my tyres.

A couple of years ago I put in a 2" inch lift with new rear shocks, and new Goodyear Silent Armors 245/75R16.

Since then I have suffered with what I thought was very harsh suspension.

Anyway, took on the advice here, and dropped pressures from 46r and 40f, to 38r and 28f. The difference in driving comfort is amazing, although it does affect handling a little on bitumen.

Having an xrox bar, I think I might up the pressures to 40r and 30f for a little while and see how it goes.

Hi Biggin,

Based on the specs of those tyres you should try either 32F/40R (+2psi) or 34F/42R (+4psi). The 28/38 applies to tyres with a similar load rating and max pressure to the standard tyres.

I've just had a go at measuring the contact patch length on my tyres. At 30.5psi the length of the front contact patch is 185mm. The rears have 39psi in them (warmed slightly after a drive to the shops) and with an almost empty fuel tank the contact patch is 165 mm. I reckon a full tank would bring that to closer to 180mm.

cheers
Paul

one of the most important effects of tyre pressure is the ability of the vehicle to point or turn. This is referred to as understeer or oversteer. I would never put 28/38 fr/rear in any car I would choose to drive. If the rear pressure is proportionately higher than the front, you are going to get bad understeer. With ace on a D2 you can get it to handle so well that you will shock other drivers with its ability. (I like to do this in the Adelaide hills whenever I'm there) but it wont happen when you've got the rears pumped up 10+psi higher. In fact ( a real fact, not an opinion) the D2 will handle better with ace with 2-4 psi higher front pressures. so unloaded normal driving mine are usually set 42 front/38-40 rear. If you doubt this for any reason go to your local roundabout with your 28/38 pressures and see how it feels...on and off throttle......then go and adjust your pressures to 42/40 and try it.....you should be amazed at the improvement. It also improves braking. If you try what I suggest you should not have any argument with me. Just adjust rear pressure if/when you carry a load

I just replaced my Cooper ATR 235/70s with Mickey Thompson STZs the same size, and the tyre place set them at 45psi. The steering was light and I didn't quite like it, so have dropped them down to 40 psi all round today, but will keep experimenting, I used to run 36 with the ATRs. Some great technical stuff on this thread .

Jeff

Also,with the D2,does anyone know which axle has the most weight,unloaded?
I recon it maybe the front,many people assume it is the rear.
That is a TD5 i am thinking about.The V8 would be less at the front i presume.

Originally Posted by ramblingboy42

If the rear pressure is proportionately higher than the front.

I think you have missed the point. The method of determining inflation on the basis of axle loading ensures that the both front and rear tyres are inflated to the same degree relative to the loading.

Funny I should come across this thread.
I have had my D2 for three days. today I checked the pressures they were all over the place.
I got the owners manual out and it stated black top unladen, 28 psi front 38 psi rear.
Are you saying I should up this and if so what to.
I'm not going off road yet.
Regards Nick ( worane)

so, OffTrack, you are telling me that at placarded tyre settings of 28/38 that the vehicle will handle neutrally? The front will wash out like a bucket of **** when pushed at those settings.....as I said before regardless of your basis of axle loading, go and try it on your nearest roundabout......dont argue with me here.....I know I'm right. If anyone wants to accompany me through a series of proof tests I am happy to accommodate them.